This invention relates generally to a method of chemical analysis and particularly to a method for characterizing the curing of an epoxy.
The structure and properties of epoxies are known to strongly depend on the extent of cure and physical aging which has taken place after the cure cycle is completed. A number of physicochemical techniques have been used or developed toward a better characterization of cure and physical aging phenomena in epoxies. Among them are such techniques as FT-IR spectroscopy, described by M. K. Antoon et al. in Polym. Comp., 2, 81 (1981); thermal analyses described by G. L. Hagnauer et al. in ASC Symp. Ser., 221, 229 (1983); GPC (size exclusion chromatography), described by G. L. Hagnauer et al. in ASC Symp. Ser., 227, 25 (1983) and ASC Symp. Ser., 221, 193 (1983); microdielectrometry, described by N. F. Sheppard et al. in Proc. of the 26th SAMPE Symposium, Los Angeles, 65, (1981); torsional braid analyses described by J. B. Enns et al. in ACS Symp. Ser., 203, 27-63 (1983); .sup.13 C solid-state NMR, described by A. N. Garroway et al. in Macromolecules, 15, 1051 (1982); thermally stimulated current measuement, described by T. D. Chang et al. in Polym. Eng. Sci., 22, 1213 (1982); fluorescence, described by R. L. Levy et al. in ACS Org. Coat. Appl. Polym. Sci. Proc., 48, 116 (1983) and Wang et al. in ACS Polym. Mater. Sci. Eng. Proc., 49, 138 (1983); and ESR spectroscopy, described by A. Gupta et al. in J. Appl. Polym. Sci., 28, 1011 (1983).
While these experimental techniques provide useful information on the extent of cure and on epoxy structure, there are certain limitations and disadvantages associated with each technique. For example, FT-IR fails to monitor later stages of cure when the epoxy peak disappears. The use of GPC and of size exclusion chromatography is limited to the early stages of the curing reaction. The fluorescence techniques measure the emission of the epoxy as a function of increasing viscosity, not the formation of reaction products. ESR techniques also primarily measure decreasing mobilities of the label with increasing cure and viscosity but provide only limited information on the reaction products. For example, Brown and Sandreczki in Macromolecules, 16 1980 (1983) observed different ESR spectra which may allow spectroscopic monitoring of the initial addition products of the reaction of an epoxy with a nitroxide monoamine, if the reactivity of the nitroxide is similar to that of a diamine.
.sup.13 C MAS-CP (magic angle spinning-cross polarized) solid-state NMR is another technique for the characterization of polymeric solids. As Garroway et al. demonstrated, it can provide information on certain molecular motions in epoxy. It is not easy, however, to obtain quantitative compositional information on the reaction products by .sup.13 C NMR in cross-linked polymers due to such complications as line broadening and spinning side bands. The fact that the peak intensity is not generally representative of the concentration in crosspolarization experiments is another problem. .sup.15 N NMR, in principle, may be more useful, but its sensitivity is poor because of the low natural abundance.
The article entitled "A New Method to Characterize Curing of Epoxy with Aromatic Diamines by Azochromophore Labeling" by In-Joo Chin and Chong Sook Paik Sung in Macromolecules 12, 2603-2607 (1984) describes a new method to obtain quantitative compositional information on the curing of epoxy with aromatic diamines by azochromophore labelling. In this technique, a small amount of an azochromophore, such as p,p'-diaminoazobenzene (DAA), which has reactivity similar to the curing agent, such as diaminodiphenyl sulfone (DDS), is used to provide an indication of the extent of cure. As the epoxy is cured, .lambda..sub.max of the .pi..fwdarw..pi.* transition corresponding to the azo bond of the curing agent is red-shifted in a way that provides spectral discrimination for the cure products (cross-linkers, branch points, linear chains, chain ends and unreacted diamines). The accuracy of the compositional analyses depends on the proper assignments of .lambda..sub.max positions and the determination of the extinction coefficients for the various cure products.
Epoxies formed with aromatic diamines such as DGEBA-DDS (diglycidyl ether of bisphenol A-diaminodiphenyl sulfone) are important for high-temperature applications. In the DGEBA-DDS system, at the stoichiometric ratio, there are five major species during curing. This is because the amine groups are the major reactive species, and the hydroxyl groups do not initiate epoxy-polymerization in uncatalyzed epoxy cured with aromatic diamines at the stoichiometric ratio.
In order to understand cure mechanisms, kinetics, and structure/property correlations, it is important to know the relative concentrations of the primary reaction products during the course of the curing process.
It is therefore an object of the present invention to provide a means for determining the extent of cure in applications utilizing epoxies of varying thickness and in particular on metallic substrates in situ.
It is another object of the present invention to provide a process for determining the relative concentrations of the primary reaction products produced during the curing of epoxies.
It is yet another object of the invention to provide a sensitive method for determining the reactivity ratios, the activation energy, and the weight-average molecular weight of the reaction products produced during the polymerization of the epoxy.